Methacrylates coatings that may be used for ePTFE surfaces do not necessarily translate well to polypropylene. The underlying reason behind the inability of the methacrylate coatings to function properly as polypropylene coatings as compared to ePTFE coatings is due to the several orders of magnitude difference in surface area between the two substrate materials. ePTFE has an increased surface area over non-porous (non-expanded) material due to the nature of being porous and having multiple nanoscale fibrils interconnecting micron-scale nodes. Polypropylene has a very small surface area in comparison due to being made of consolidated smooth monofilaments which have thicknesses on the order of hundreds of microns to a millimeter. Because of this large reduction in surface area polypropylene coatings need be orders of magnitude thicker in order to accommodate drug quantities that will be clinically effective. In other words, if the surface area of polypropylene is 1,000 times less than ePTFE (from a bulk area perspective, such as a per square inch of mesh) then a coating holding the same amount of drug per polymeric coating mass will need be 1,000 times thicker to provide equal drug loadings in terms of drug density per bulk mesh area. The methacrylates coating formulations with a biphenyl center result in bulk polymers being brittle and having high bending stiffness. These mechanical properties are not an obstacle when coated on the ePTFE as coatings are of nanoscale, and the mechanics of nanoscale materials are not the same as that of their bulk material counterparts. When the thicker coatings are applied to polypropylene the brittleness of the materials cause the coatings to crack and break off of the monofilaments. Methacrylates also have processing and biocompatibility issues such as low degrees of polymerization, oxygen inhibition, slow kinetics, monomer toxicity, free-radical chemistry with very reactive heads capable of side reactions with drugs and solvents, susceptibility to Trommsdorff Effect, higher initiator to monomer ratios, and others. A polymeric coating that may be used effectively for both ePTFE and polypropylene surfaces is thus desired.
ePTFE is a general name given to any multitude of PTFE sheet or rod materials which have been mechanically deformed through tension, resulting in unique microstructures which translate into the unique material properties and applications of ePTFE materials. The physical attributes of these microstructures (node shape, node size, porosity, and internodal distance) result in varying degrees of the inherent low surface energy of PTFE. Unidirectional pulling of PTFE results in a microstructure characterized as narrow, broad (disc-like) nodes which continue into the depth of the microstructure and are interconnected by parallel running fibrils which run perpendicular to the planes of the nodes. Multidirectional (typically two perpendicular directions within the plane defined by the continuum of an ePTFE sheet) and radial pulling of PTFE materials result in spherical-like, isolated nodes which are interconnected by fibrils which run in all directions, reaching out from the curved surfaces of the node structures. FIG. 1 demonstrates typical microstructures formed from the two different pulling methods which result in comparable internodal distances with different node structures, spherical (multidirectional) and narrow, broad nodes (unidirectional). The spherical nodes have diameters similar to that of that of the thickness of the narrow, broad nodes.
While the customary coating application methods such as dipping, passive wicking, mechanical deformation assisted wicking, brushing, and spraying may generally be used for coating of unidirectional pulled ePTFE, these coatings methods when attempted on multidirectional pulled ePTFE similar to that shown in FIG. 1 may result in poor penetration of the casting solution into the microstructures and surface build-up and clogging of the pores. This lack of penetration and surface coating issue may often be detectable with the naked eye. In FIG. 1 it may be seen that the multidirectional pulled samples exhibit increased fibril density and hence are more challenging to penetrate within the depths of the low surface energy ePTFE (which is more prevalent in the multidirectional pulled ePTFE due the nature of the resulting microstructure). In short, the increased PTFE surface area in the multidirectional pulled ePTFE results in an increased resistance to penetration. In order to obtain similar fibril density that would coat with the traditional means further multidirectional pulling would be required and nodes would become smaller, fibrils thinner, and internodal distances would increase. Further pulling of this material would compromise its mechanical properties as a result of increased plastic deformation, limiting or excluding the use of the materials for the intended application. Thus it is desirable to have coating application methods that may be effectively used with a variety of ePTFE surfaces, including the multidirectionally pulled ePTFE samples.
Certain drugs, especially those containing amine groups, are often susceptible to degradation during free radical chain growth polymerization reactions, due to hydrogen abstraction between the amine and the living polymer's radical end. Polymerization is terminated and the drug molecule is degraded. Loading these drugs on polymeric coatings by polymerization of the coating in presence of drugs may result in significant loss of drug due to degradation. Further, the drug degradation products are often included within the resulting coating's polymer network. Thus, there is a need for improved methods of loading drugs onto the polymeric coatings.